Abstract not available

In this thesis, the implementation and results of studies into the effect of pH on the kinetics of various iron transformations in natural waters are described. Specific studies include i) the oxidation of Fe(II) in the absence and presence of both model and natural organic ligands, ii) the complexation of Fe(III) by model organic compounds, and iii) the precipitation of Fe(III) through the use of both laboratory investigations of iron species and kinetic modeling. In the absence of organic ligands, oxidation of nanomolar concentrations of Fe(II) over the pH range 6.0 -- 8.0 is predominantly controlled by the reaction of Fe(II) with oxygen and with superoxide while the disproportionation of superoxide appears to be negligible. Oxidation of Fe(II) by hydrogen peroxide, back reduction of Fe(III) by superoxide and precipitation of Fe(III) have been shown to exert some influences at various stages of the oxidation at different pH and initial Fe(II) concentrations. In the presence of organic ligands, different effects on the Fe(II) oxidation kinetics is shown with different organic ligands, their initial concentrations and with varying pH. A detailed kinetic model is developed and shown to adequately describe the kinetics of Fe(II) oxidation in the absence and presence of various ligands over a range of concentrations and pH. The applicability of the previous oxidation models to describe the experimental data is assessed. Rate constants for formation of Fe(III) by a range of model organic compounds over the pH range 6.0 -- 9.5 are determined. Variation of rate constants for Fe(III) complexation by desferrioxamine B and ethylenediaminetetraacetate with varying pH is explained by an outer-sphere complexation model. The significant variation in rate constants of Fe(III) complexation by salicylate, 5-sulfosalicylate, citrate and 3,4-dihydroxylbenzoate with varying pH is possibly due to the presence of different complexes at different pH. The results of this study demonstrate that organic ligands from different sources may influence the speciation of iron in vastly different ways. The kinetics of Fe(III) precipitation are investigated in bicarbonate solutions over the pH range 6.0 -- 9.5. The rate of precipitation varies by nearly two orders of magnitude with a maximum rate constant at a pH of around 8.0. The results of the study support the existence of the dissolved neutral species Fe(OH)30 and suggests that it is the dominant precursor in Fe(III) polymerization and subsequent precipitation at circumneutral pH. Variation in the precipitation rate constant over the pH range considered is consistent with a mechanism in which the kinetics of iron precipitation are controlled by rates of water exchange in dissolved iron hydrolysis species.

abstract / Earth Sciences / Doctoral / Doctor of Philosophy

M. Tech. Chemical Engineering / The lack of clean and fresh water has become a worldwide problem because of water pollution caused by industrialization. Contamination of natural water sources by heavy metal is a worldwide public health problem, leading to waterborne outbreaks of infectious hepatitis, viral gastroenteritis, and cancer. Therefore it very important to remove these toxic metal ions from municipal and industrial effluents in order to protect plants, animals and human beings from their adverse effect before discharging into natural water bodies. Although, several separation methods such as filtration, reverse osmosis and membrane technology have been developed to remove these toxic heavy metal ions from wastewater, however these conventional treatments technologies were found to be expensive on a sustainable basis. Adsorption process was identified as the most effective, and extensively used essential process in wastewater treatment, and in order for adsorption process to feasibly remove pollutants from wastewater, there should be a need for a suitable adsorbent which will have a large porous surface area, and a controllable porous structure. Through the application of nanotechnology, nano adsorbents can be developed as effective adsorbents to treat wastewater. The main objective of this project was to apply magnetic metal doped iron oxides as an efficient adsorption media for the removing of Cr(VI), Cd(II) and V(V) ions from wastewater.

This thesis describes studies of materials which can be exploited for hydrogen production from water and sunlight. The materials investigated are maghemite (γ-Fe2O3), magnetite (Fe3O4) and especially hematite (α-Fe2O3), which is an iron oxide with most promising properties in this field. Hematite has been deposited using Atomic Layer Deposition (ALD) - a thin-film technique facilitating layer-by-layer growth with excellent thickness control and step coverage. The iron oxides were deposited using bis-cyclopentadienyl iron (Fe(Cp)2) or iron pentacarbonyl (Fe(CO)5) in combination with an O2 precursor. Since it is crucial to have good control of the deposition process, the influence of substrate, process temperature, precursor and carrier gas have been investigated systematically. By careful control of these deposition parameters, three polymorphs of iron oxide could be deposited: hematite (α-Fe2O3), maghemite (γ-Fe2O3) and magnetite (Fe3O4). The deposited materials were characterized using X-ray Diffraction, Raman and UV-VIS Spectroscopy, and Scanning Electron Microscopy. Hard X-ray Photoelectron Spectroscopy (HAXPES) was also used, since it is a non-destructive, chemically specific, surface sensitive technique – the surface sensitivity resulting from the short mean escape depth of the photoelectrons. The depth probed can be controlled by varying the excitation energy; higher photoelectron energies increasing the inelastic mean-free-path in the material. HAXPES studies of atomic diffusion from F-doped SnO2 substrates showed increased doping levels of Sn, Si and F in the deposited films. Diffusion from the substrate was detected at annealing temperatures between 550 °C and 800 °C. Films annealed in air exhibited improved photocatalytic behavior; a photocurrent of 0.23 mA/cm2 was observed for those films, while the as-deposited hematite films showed no photo-activity whatsoever. The optical properties of low-dimensional hematite were studied in a series of ultra-thin films (thicknesses in the 2-70 nm range). The absorption maxima were shifted to higher energies for films thinner than 20 nm, revealing a different electronic structure in thin films.

No description available.

In this thesis, the implementation and results of studies into the effect of pH on the kinetics of various iron transformations in natural waters are described. Specific studies include i) the oxidation of Fe(II) in the absence and presence of both model and natural organic ligands, ii) the complexation of Fe(III) by model organic compounds, and iii) the precipitation of Fe(III) through the use of both laboratory investigations of iron species and kinetic modeling. In the absence of organic ligands, oxidation of nanomolar concentrations of Fe(II) over the pH range 6.0 -- 8.0 is predominantly controlled by the reaction of Fe(II) with oxygen and with superoxide while the disproportionation of superoxide appears to be negligible. Oxidation of Fe(II) by hydrogen peroxide, back reduction of Fe(III) by superoxide and precipitation of Fe(III) have been shown to exert some influences at various stages of the oxidation at different pH and initial Fe(II) concentrations. In the presence of organic ligands, different effects on the Fe(II) oxidation kinetics is shown with different organic ligands, their initial concentrations and with varying pH. A detailed kinetic model is developed and shown to adequately describe the kinetics of Fe(II) oxidation in the absence and presence of various ligands over a range of concentrations and pH. The applicability of the previous oxidation models to describe the experimental data is assessed. Rate constants for formation of Fe(III) by a range of model organic compounds over the pH range 6.0 -- 9.5 are determined. Variation of rate constants for Fe(III) complexation by desferrioxamine B and ethylenediaminetetraacetate with varying pH is explained by an outer-sphere complexation model. The significant variation in rate constants of Fe(III) complexation by salicylate, 5-sulfosalicylate, citrate and 3,4-dihydroxylbenzoate with varying pH is possibly due to the presence of different complexes at different pH. The results of this study demonstrate that organic ligands from different sources may influence the speciation of iron in vastly different ways. The kinetics of Fe(III) precipitation are investigated in bicarbonate solutions over the pH range 6.0 -- 9.5. The rate of precipitation varies by nearly two orders of magnitude with a maximum rate constant at a pH of around 8.0. The results of the study support the existence of the dissolved neutral species Fe(OH)30 and suggests that it is the dominant precursor in Fe(III) polymerization and subsequent precipitation at circumneutral pH. Variation in the precipitation rate constant over the pH range considered is consistent with a mechanism in which the kinetics of iron precipitation are controlled by rates of water exchange in dissolved iron hydrolysis species.

Plant residues added to soil are a source of nutrients for plants and soil organisms and increase soil organic matter which has an important role in improving soil structure and fertility, hence maintaining soil quality for sustainable agriculture. In order to utilize plant residues for increasing soil organic matter more effectively, it is necessary to understand the mechanisms of plant residue decomposition. Soil organic matter decomposition is influenced by several factors such as plant residue quality, temperature, water availability, soil structure and soil texture, particularly clay content. The interaction of clay and decomposition of organic matter has been studied in the past. Nevertheless, many studies investigated this interaction in natural soil or under field conditions over long periods of time. Variation in environmental factors may influence the interaction of clay and decomposition of organic matter, thus in most previous studies their effect cannot be separated from the direct effect of clay on decomposition. To study the direct effect of clay on organic matter decomposition, four experiments with different objectives were carried out using isolated natural clay, under controlled conditions (e.g. temperature and organic matter input) and a short incubation period (approximately one month). All experiments were carried out using a sand matrix to which different clay types, clay fractions (natural or with iron oxide partially removed) or clay concentrations were added together with mature wheat straw (C/N 122 in most experiments, except Experiment 2 where the wheat straw had a C/N of 18) and a microbial inoculum. To investigate the effect of clay type, two clay types were added. They were isolated from Wiesenboden (W) and Red Brown Earth (RBE) soil. Clay types from both soils contained kaolinite and illite, but smectite only occurred in W clay. Iron oxide is thought to be important for the binding of organic matter to clay, therefore two clay fractions were used, the clay with native iron oxide (natural clay) and clay from which iron oxide was partially removed by citrate-dithionite-bicarbonate treatment (citrate-dithionite clay, CD clay). The following parameters were measured: pH, water loss, respiration rate, microbial community structure using phospholipid fatty acid analysis and, in some experiments, particulate organic matter. In all experiments, the water content of the substrate mixes was adjusted only at the start; water loss was greatest in the control and decreased with increasing clay content. The aim of the first experiment was to study the effect of the concentration of W clay on decomposition of wheat residues. Respiration (i.e. decomposition of the wheat straw) was affected by clay in two ways (i) decreased decomposition, thus protection of organic matter, in the initial phase at all concentrations (5, 10, 20 and 40%) and throughout the incubation period at ≤ 20% clay, and (ii) greater water retention at higher clay concentration particularly 40% clay that allowed maintenance of higher respiration rates towards the end of incubation. Generally, clay concentration had an effect on microbial community structure but not on microbial biomass. The effect of clay concentration was also investigated in the second experiment, but using RBE clay and a narrower range of concentrations (0, 2.5, 5, 10 and 20% clay) than in the first experiment with W clay. The wheat residue used in this experiment had a lower C/N ratio compared to the other three experiments (C/N 18 compared to 122). In contrast to the first experiment, cumulative respiration of the clay treatments was greater than that of control throughout the incubation, thus clay increased rather than decreased decomposition. This may be due to the properties of the wheat residue used in this experiment which contained more water-soluble compounds, the diffusion of which would be enhanced in treatments with clay compared to the control due to their higher water availability. However, considering only the treatments with added clay, cumulative respiration followed the same pattern as in the first experiment, with highest cumulative respiration at 20% clay. In general, microbial community structure, microbial biomass and microbial groups (i.e. bacterial and fungal fatty acids) were affected by the presence of clay and sampling time, but there was no clear relationship between these factors and the richness and diversity of the microbial community. The aim of the third experiment was to determine the effect of clay concentration (5 and 40% of W clay) and fraction (natural or citrate-dithionite clay) on decomposition of wheat straw and microbial community structure. Clay fraction and concentration strongly affected the respiration rate and microbial community structure as well as microbial biomass but not the concentration of particulate organic matter (POM). Compared to the control, partial removal of iron oxide strongly increased decomposition at both concentrations whereas clay with iron oxides reduced the decomposition. Microbial community structure was affected by clay fractions, particularly at 40% clay. The aim of the fourth experiment was to determine the effect of clay fraction (natural and citrate-dithionite clay) and clay type (W clay or RBE clay) at 5% clay on decomposition of wheat straw and microbial community structure. Clay type and the partial removal of iron oxide had a significant effect on the decomposition rate but did not affect POM concentration. As in the third experiment, partial removal of iron oxide increased respiration rate, the effect was less pronounced in RBE clay than in W clay. Clay type and fraction strongly affected microbial community structure. In conclusion, the experiments showed that native clay generally reduces organic matter decomposition by binding and occlusion. The importance of iron oxide for the protective effect of clay on organic matter decomposition was shown by the fact that partial removal of iron oxide strongly increased decomposition rate compared to the native clay. The two clay types differed in their effect. The W clay containing smectite protects organic matter to a greater extent than RBE clay with predominantly illite and kaolinite due to its higher surface area and CEC that lead to binding and or occlusion. The results also showed that although clay reduces organic matter decomposition under optimal water availability, this effect can be reversed as the substrates dry out because the greater water retention of substrates with clay concentrations > 10% compared to the pure sand matrix allows maintenance of a greater microbial activity. Clay type, fraction and concentration affected microbial community structure via their effect on organic matter and water availability. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1521949 / Thesis (M.Ag.Sc.) -- University of Adelaide, School of Earth and Environment Science, 2010

Thesis (Ph. D.)--University of North Texas, May, 2007. / Title from title page display. Includes bibliographical references.

Thesis (M.S.)--University of North Texas, May, 2008. / Title from title page display. Includes bibliographical references.